Caverns of Darkness is a 12-level mapset for Doom 2 by The Chaos Crew in 2002, using a custom source port based on the Eternity Engine v3.29 called the "COD Engine". Because of the custom engine, MS-DOS or a DOS emulator such as DosBox is required to play. The modification is well known for its liberal use of fake three-dimensional constructs using middle textures, and ambient sounds.

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In 2008, Graf Zahl released a patch for Caverns of Darkness, allowing it to be played using ZDoom. It works by substituting DECORATE actors in place of the original hard-coded custom thing types and weapons, as well as using MAPINFO to use a custom map translator to account for the COD engine's new sector and linedef types.